This is a most confusing season. I hear many of you talking about these holidays, and while it frustrates me to not understand, I have done my best to piece it together.
So far as I can tell... this day celebrates the birth of a great warrior called Christ, who was so powerful and fierce that even death could not conquer him. He apparently had help from his teammates such as Saint Nicholas (who operated under the code-name of 'Santa Claus'), who's expertise in stealth allowed him to effortlessly enter the homes of mortals, leaving gifts for children and destroying Christ's enemies with his army of magical reindeer.
I believe there are also members of this team including an immortal man made of snow who regenerates every winter, but I have not sussed out the details of them yet.